Cathodic protection system and anode therefor



Sept- 3, 1957 H. A. ROBINSON ET A1. 2,805,198

CATHODIC PROTECTION SYSTEM ANDYANODE THEREFOR Filed Feb. 29, 1956 mmw Nk0 T.

QQQN uw tou United States Patent CATHODIC PROTECTIN SYSTEM AND ANUDETHEREFOR 1 Harold A. Robinson, Midland, Mich., and .lohn J. Newport andOliver Osborn, Lake Jackson, Tex., assigner-s to The Dow ChemicalCompany, Midland, Mich., a corporation of Delaware Application February29, 1956, Serial No. 568,6i9

8 Claims. (Cl. 204-197) This invention relates to a cathodic protectionanode formed of a galvanically active metal and to a cathodic protectionsystem wherein such a protection anode is employed. Y

Cathodic protective anodes have been used for many purposes, such aspreventing corrosion of structures in sea water, pipelines buried in thesoil and internal areas of hot water heaters. For the most part, theseanodes are made of readily expendable magnesium which has been alloyedwith other metals to give the anode properties which particularly adaptit for its intended use. For example, cell magnesium has a satisfactorysolution potential, but a poor current eciency. Certain magnesium alloyshave improved current eticiencies, but lower solution potentials thancell magnesium. Consequently, more alloy anodes are required to protecta given structure, but under predetermined conditions of use theseanodes will have a longer useful life. Offsetting to a certain extentsome of the advantages of these anodes made from magnesium alloy is thecost of the additional process steps and alloying ingredients.

It is an object of this invention to provide an improved cathodicprotection anode composed essentially of magnesium and containing minoramounts of manganese and aluminum present in a predetermined ratio toeach other. Another object of this invention is to provide an improvedsystem for cathodically protecting structures immersed in media in whichthey are subject to corrosion. Still another object of this invention isto provide a process for making an improved cathodic protection anodedirectly from the melt in cell in which the magnesium metal is rstprepared.

It has been discovered that an improved cathodic pro tection anode isobtained by forming the anode from a magnesium base alloy containingfrom 0.50 to 1.3 percent by weight manganese and not more than 0.010percent by weight aluminum. A preferred method of making these anodes isto add a suitable manganese cornpound to the feed of a magnesiumelectrolytic cell, and the preferred composition comprises from 0.50 to.80 manganese and not over 0.005 aluminum (in weight percent), with thebalance essentially magnesium. When such anodes are employed in a systemfor cathodically protecting structures composed of a metal less anodicthan'magnesium, they have a greatly improved current eiciency combinedwith an unusually high solution potential. These systems are thusadvantageous in that they'require fewer anodes and have a longer lifethan cell magnesium anodes, for example.

The present invention is useful in providing cathodic protection forstructures immersed in sea water, and can be vused under some conditionsas a protective anode in hot water heaters. Yet anodes and systemscomprisly fall between 200 and 300 hours per pound. Anodes at a givenvoltage or potential. Consequently, .an anode. metal having relativelylow solution potential is desir- L able to avoid useless currentgeneration which is in turn accompanied by rapid consumption of theanode metal.

When the structure to be protected is buried in high resistance soil,this high resistance so Vreduces the current owingthrough the soil fromthe anode to the corrodible structure thatV a high voltage oranodesolution potential is needed to adequately protect the stiucture.The all-around adaptability of the anode and the cathodic protectivesystem comprising the present invention ismanifested Vby the, fact thatit retains its high current eiciency and solution potential in seawater, salinersoils and non-saline soils where the anode usuallyoperates in a sulfate rich backfill.

Anodes comprising the present invention have the following compositionin percent by weight:

Magnesium; At least l98.5. Manganese 0.50 to 1.3. Aluminum Not over0.010. Iron .001 to 0.03. Tin Not over 0.01. Nickel Not over .002. LeadNotover 0.01. Other metals (each) Not over 0.05.

In the electrolytic production of magnesium, the molten magnesium in theelectrolytic cell is referred to as cellmagnesium. Essentially, cellkmagnesium is pure magnesium, but certain adventitious elements arepresent in small amounts. In .thel production of anodes comprising thepresent invention, amounts of these adventitious elements must becontrolled. Iron is present in at least 0.001 percent by weight and mustbe limited to a maximum of 0.03 percent if an anode of the desirablecharacteristics of the present invention is to be obtained. Nickel isrestricted toy not over 0.002 .percent by weight and the tin and leadVshould rnot exceed 0.01 percent. The other metals present do not exceed0.05 percent each. The total of all such adventitious elements does notexceed 0.2 percent in the cell magnesium.

Aluminum is always present in small amounts in cell magnesium and willvary from 0.0001 up to 0.02 weight percent. Although it has previouslybeen recognized that high percentages of iron were detrimental in anodesused for cathodic protection, little if any attention was directed tothe amount of aluminum in the cell magnesium.

' amount of aluminum present, an alloy is produced which,

when made into anodes, combines an even higher solution potential thancell magnesium with a high currentY .efliciencv Measuring currenteiciency in ampere hours per pound of anode metal, cell magnesium anodesusualcomprising the present invention, on the other hand, will It hasnow vbeen found that the current eiciency of anodes made from cellmagnesium is closely related to the amount of aluminum present. Withlarger amounts of aluminum, i. e. over .O05 Weight percent,

have current eici'encies of at least 400 and as high as H565 hours` perpound: Although the exactv reason for this remarkableincrease in currenteliciency is not known, it is believed that the manganese in some wayties up or counteracts the detrimental eiects of the ironaluminum phasewhich is known to adversely affect the current elliciency of the anodes.

The solution potential of anodes comprising the present invention issubstantially uniform wheuthe manganese solutionpotentials doesdecrease. In-thisI samerange, thel amount of manganese isadjustedrelative' to the amount of aluminum` present toxobtainenhancedcurrent efficiency and the highest solutionpotentials, Figure lillustratesthe. critical effectof the Mn/Al ratio on the currenteiciency1ofthe anode by plot-tingxtest data from alloys comprising thepresent invention which* have similar aluminum content on a graph inwhichfthe ordinates represent ampere hoursl per pound of'anode metal andtheabscissas `represent the percentby weight of manganese. Thedetails-of thel tests and data` upon which-Figure l is-basedV are setforth in Examples ll and 2 below.

It is apparent from Figure l that the low aluminum alloys (less than.001 percent) have consistently good current efficiencies when between0.5 percent and 1.3 percent by weight of manganese is'present. As theamount ofv aluminum increases, the decrease in current eiliciencybecomesmore pronounced in the lower ranges of manganese content. Higherpercentages of manganese result in satisfactory efliciencies for allaluminum contents not over .01%. to aluminum is the critical factor.Since itris desirable to know how little manganese can be added andstill overcome the detrimental eifect of the aluminum, an

lt is thus seen that the ratio of manganese of the' examples wereobtained under conditions simulat- The-preferredi manner 'of producingsuch alloyed anodes,

however, comprises adding a manganese compound, such as manganesedioxide or manganese dichloride, to the feed of an electrolytic cell formaking magnesium. The constituents of the cell are analyzed at frequentintervals and the manganese added in the feed is adjusted according. tothe aluminum content of the cell magnesium. In this connection it isimportant to note that manganese is an, expensive alloying metal andminimum amounts useable areemploycd to' reduce the cost of thesesacrificial anodes which maybe employed tothe extent of several tonsforuses such as protecting pipelines.r The casting of anodes from thisalloy follows. the conventional procedures althoughv it is preferred toemploy Arelatively cool molds. Y

In order to enable those skilled in the art to more readily-practice thepresent invention, the following examples are set forth to illustratethe present invention:

. ing a composition as set forth above, exceptf'or the varia-Y Example 1The melt in a magnesium cell was analyzed by a direct readingspectrophotometer and found to contain by weight, .0052 percent Al,.0013 percent Cu, .014 percent Fe, less than .0l percent Ca, less than.0005 percent Ni, less than .001 percent Pb, less than .001 percent Si,less than .0l percent Sn, less than .02 percent Zn, and the balancemagnesium. To ther feed in this cell was added enough manganese dioxideto produce in the melt a minimum of 0.81 percent by weight of manganese(0.5-l-60(.0052) =0.5-{-0.3l). Final analysis showed 0.92 Weight percentof manganese.

From this melt, magnesium `anodes used for laboratory investigationswere cast in iron molds just slightly over C. and immediately air cooledto room temperature. Y

These anodes were prepared bysectioning specimens roughly 7 inches longand 1/2` inch square cross section from l7 pound D-shaped productionanodes. These specimens were machined togive'small rods,.5/16 inch indiameter by 6 inches long. A 2 inch section at one end of each rod wasthreaded and'a at was filed near the middle of the threaded section toallow for the stamping of identification numbers. After weighing, thethreaded vsection was smeared lightly with petrolatum grease and theanode` was then screwed into atransparent sleeve threaded at the lowerend to take the anode. The 4'inch length of anodeextending beyond thegrease-sealed sleeve was then vapor-degreased with trichlorethylene.

Employing the. anodes so prepared, experimental cellsv were assembledusing a 5.5 inch length of 3.5 inch. standardpipe for the cathode, andan` aqueous solution saturated with CaSOi and Mg(OH)2 interposed betweenthe rodandthe cathode in the electrolyte. This cell was then: connectedin series with other test cells and operated at an anode currentdensityv of 36.milliamperes per square footfor 14 days. The anodes werethen removed from the cells andthe loose corrosion product was brushedor rubbed off in a stream of runningwater. This wasv followed byimmersion in agitated, aqueous 20 percent chromic acid solution,containing` l percent AgNOs, to. remove residual corrosion products,after which the anodes were rinsed, dried and reweighed. Current e-4ciencies were calculated from the weight loss andthe quantity of currentpassed. On vthis basis, this anode showed a current eciency of 521ampere hours, per pound of anode-metal.

Example 2 A series of tests following the procedures above outlinedweremade andthe results of these tests formed the basis for Figure 1..The alloy in each case contained approximately .0005 percent Cu, lessthan .01 percent Ca, less'than .0005 percent Ni, less than .001 percent'Pb,` less than .001 percent Si, less than .0l percent Sn andV less than.02 percent Zn, with the balance magnesium 'Y except for aluminum, ironand manganese `as tabulated.

In addition to the current eflciency test above outlined' in4 Example l,these same test anodes were tested for solution potential by immersion,following the current efficiency test, in an aqueous solution saturatedwith CaSO4 and Mg(OH)2, and allowed to soak for one hour. Y

shows a number of alloys tested both for current eiiciency and solutionpotential, each of these alloys havtions in the amount of aluminum, ironand magnanese as noted. To parallel Figure l, the data, is arrangedaccording to theY aluminum content, each set of data con,- stituting anaverage of three test specimens. Alloys falling outside the prescribedrange of manganese, have Percent by Weight of metms Amp. Open Cir- Y YHours per cuit solug pound of tion poten-V Percent Percent Percent Anodetial ref: A1 Fe Mn Metal calomel 0005 008 70 557 1. 665 0005 013 84520 1. 647 0005 012 86 516 1. 644 0005 0116 99 449 1. 622 0006 0107 l.20 523 1. 664 0007 0091 80 543 1. 654 0008 0163 77 536 1. 646 0008 005084 548 1. 646 0008 0055 85 529 1. 624 0008 0118 87 479 1. 653 0008 00590 563 1. 672 0008 0059 92 532 1. 649 0008 0107 97 544 1. 654 0009 .01061 468 1. 629 0009 010 73 518 1. 650 0009 009 78 503 1. 672 0009 0145 80527 l. 638 .0009 0086 84 520 `1. 655 0009 0062 86 540 1. 633 0010 006685 550 1. 656 0011 017 86 520 1. 634 0011 0086 86 486 1. 638 0012 009481 520 1. 639 0013 016 45 355 1. 628 0013 014 74 483 1. 635 0013 0074 76498 1. 642 .0013 007 77 542 1. 675 0013 .0072 82 538 1. 655 0014 0084 97498 l. 647 0015 015 .68 460 1. 666 0016 020 64 400 1. 637 0016 012 69461 1. 628 0016 013 72 495 1. 661 0016 009 85 489 1. 635 0016 0074 91528 1. 641 0017 0076 74 498 1. 633 0017 0046 1.01 560 1. 644 0019 018 72448 1. 634 0019 017 74 507 1. 654 0019 0067 82 .486 1. 628 0019 0096 1.32 548 1. 642 0020 014 c 81 527 1. 641 0022 015 65 443 1. 634 0022 010772 450 1. 644 0023 012 68 438 1. 627 0023 0082 91 508 l. 640 0024 01161.20 546 1. 651 0025 005 78 413 1.668 0025 0068 1. 29 541 1. 626 0027019 77 451 1. 643 0028 0072 73 442 1. 625 0028 0113 76 402 1. 658 0028009 89 567 1. 661 0034 0064 72 398 1. 629 0034 0040 1. 06 507 1. 6240035 0096 84 466 1. 648 0035 0140 90 507 1. 629 0042 0091 96 509 1. 6520045 015 51 202 1. 668 0045 0159 76 438 l. 644 0052 014 92 521 1. 6650055 010 65 298 1. 627 0055 0098 70 347 1. 658 0067 0110 .63 223 l. 6310097 0092 l. 13 567 1. 626 03 012 1. 71 508 1. 554 03 011 2. 06 515 l.590 03 009 2. 48 450 1. 585 03 014 1.36 517 592 The data from this tablerelating to solution potential are plotted in Figure 2, whichgraphically `shows high solution potentials between 0.5 and 1.3 percentmanganese with rather sharp decreases in solution potential above andbelow these points. It is thus seen that the alloys of the above tablewhich fall within the scope of the present invention combine a highcurrent efficiency with a high solution potential.

In order to describe the cathodic protection system comprising thepresent invention, reference is made to Figure 3, which is a schematicillustration of an anode field positioned about a bare ferrous metalpipeline, in this case, a bare metal 8 inch diameter pipe which is to beprotected. The pipeline 1 is connected through an insulated electricalconductor 2 to an insulated collector wire 3, so-called because itinterconnects through lead wires 4-4 with anodes, 5-5. These anodes,which are formed from the alloy.. above described, may be of varioussizes, but are in `this'instance 4 inches by 20 inches and are spaced atleast 10 feet from the pipeline 1 and from 50 to 75 feet fromeach'iother.' These anodes operate 'most eiciently and have Va longerlife when surounded 'by a backll 6-6. For high' resistivity soils, thebackll 6V may be composed of 20 parts 'bentonite (dry, powdered), 75parts gypsum (dry, powdered) and 5 parts sodium sulfate (anhydrous).This backiill isolates the anode chemically and acts as an electrolyticbridge carrying electricity from anode to earth; V

For most installations, anodes need be used only on one side ofthe-,pipeline t`o be protected, lalthough exceptionally large pipes mayrequire anodes on both sides. Thespacing of the anodes-in the stationshown in Figure 3, as we ll as the total numbrof stations required, is afunction of the solution potential of the anode, theresistivity of thesoil and the amount of current required to protect the pipeline.` Otherfactors being equal, anodes of the present invention which have highersolution potentials than thoseofthe prior iart, may be spaced fartherapart, thus reducing the total number of anodes needed. Those rskilledin tharfwillrecognize that the distances, size of anodes, etc.A shown inFigure 3 may be varied to suit the particular conditions encountered.

It is apparent vtr'or'n the above detailed description of the inventionthat al athodic protection anode has been produced which,lwh'enlitiliz'ed Vina cathodic protection system, iS characterized byremarkable 'current efficiency combinedwitli high' solution potential.These results are obtained by restricting `theialloy composition of theanode -within verynarrow'and Icritical ranges and particularly by'maintainingthe ratio of manganese `to aluminum at a prescribedminimum.' The invention is especially adapted forluse in'highresistivitysoils, wherein a high solution potential is needed.

VvWhat is 'claimed is:

1.y A cathodicprotection'anode formed of galvanically active metalhaving the following composition in percent by weight:

the amount of manganese in weight percent being at least equal to 0.5+60(percent by Weight of aluminum).

2. A cathodic protection anode formed of a cast body of galvanicallyactive metal having the following cornposition in percent by weight:

Magnesium At least 99.16. Manganese 0.5 to' 0.8. Aluminum Not over0.005. Iron Y 0.001 to 0.03.

Nickel Not over 0.002. Tin Not over 0.01. Lead Not over 0.01. Othermetals (each) Not over 0.05.

the amount of manganese in weight percent being at least equal to0.5-|-60(percent by weight of aluminum).

3. In combination with a structure composed of corrodible metal lessanodic than magnesium immersed in a medium wherein it is corrodible, aprotective system of sacrificial metal submerged in the corroding mediumand electrically connected to said structure and comprising 7 anexpendable quantityof inagnesinnimetal;hin/ingthe`v followingcomposition in percent by Weight:

Magnesium At least 9825. Manganese 0.50. to 14.3. Aluminum f Not over0.041. Tin Not over OO 1'. Lead Not over 0101, Ii'nriv f 0.001 `to`0.03. NickeL Not over 0.002'. Other metals (each) Not over"0.05.

the percent by weightv omanganese' beingat leastequaly to 0.5.-1-60(percent byweighti of aluminum)- 4. The combination dcned in claim: 3'wherein theA corroding medium is sea waten.

5. The combination; defined in. claimk 34 wherein: the co'rro'dingmediumisasoil..

6. In combination. withv a' structureY composedioi''feraY rous metalimmersed inahighlresistii/ity-soil; aprote'ctive system ofr sacrii'cialmetal.l submerge'dfin` the? high; re sistivity soil. andl electrically.connected to Vsaid structure. so asA to form al galvanic celltherewith; and comprising an expendable 'quantity of ama'gnesiimr alloyhaving the following! comp usitio'n` in. percent by weightc MagnesiumAt.leastl98.5. Manganese 0.50 to`1'.3. v Aluminum No't over'00'1`.. Tin-Notover 0:01'. Lead Not over" ;'01". Iron 0.0011602031 Nickel Notover':0.1002; Other metals (each) Noti over 0L05".

the amount of manganese Yinweight percent-being' at: least equal to0.5-1-60 (percent by Weightiof. aluminum).

7. A method of cathodically protecting a 'structure composed of acorrodible metalless anodic thanr mag.- nesium immersed in a Ymediumwherein it iscori'odible, which comprises placing a protectivesysterh^=of sacrificial4 metal in the corroding medium composed-0fanexpendable quantity `of magnesium.` alloy'haying; the.Y followingcomposition in percent weight: f

Y seA .A the amountfof manganese in weightpercent being' at least equalto 0560 (percent byzweight of' aluminum), and

electrically-connecting said structure and the sacricial metal toproduce a galvanic cell in which the sacrificial metal is the anode andthestructure is the cathode.

8. A cathodic protection anode formed of galvanieally activemetal'having the following composition in'percent by weight: v

Magnesium At least 98.5. Manganese 0.50 to 1.3. Aluminum Not over 0.01.Tin Not over 0.01. Lead Not over 0.01. Iron 0.001 to 0.03. Nickel Notover 0.002. -Other metals i(each) No't over 0.05.

the amount of manganese in weight percent being at least equal to 02541-(percent by weight of aluminum) when the percent by weight of aluminumexceeds .007.

References Cited in the' le of this patent UNITED STATES PATENTS1,960,700 G'ann' et a1. May 29, 1934 2,431,723? Yerkes Dec. 2, 19472,478,479 Grebe et al. Aug. 9, 1949 2,645,612 Taylorf j July 14, 19532,698,230 Boyle' Dec. 28, 1954 2,742,355 Emley Apr. 17, 1956 OTHERREFERENCES H. H. Robinson, Transactions of the Electrochemical Society;V01. (1946), pp. 496-499.

1. A CATHODIC PROTECTION ANODE FORMED OF GALVANICALLY ACTIVE METALHAVING THE FOLLOWING COMPOSITION IN PERCENT BY WEIGHT: